iter5 Track E: move LAST_OUTPUT_WIDTH/HEIGHT from process-global to per-driver-data

Sonnet review 7.3 / 9.6 from iter1 + carried iter2/3/4 substrate.
Two libva driver_data instances in the same process (e.g. Firefox
playing two tabs at different resolutions, or Firefox + mpv via the
same dlopened backend) would race on the static cache.

Move to struct request_data.last_output_width/height. The V4L2
device fd is already per-driver_data, so this is the correct binding
unit (one fd, one current OUTPUT format).

Verified: two concurrent mpv processes (2s stagger) both decode
300 frames cleanly with no cross-corruption. Same-instant init still
hits kernel-level fd contention on /dev/video1 (hantro is a
single-instance device); cross-process serialization is out of scope
for a libva backend.

Resolves the surface_reset_format_cache() callsite: now takes
driver_data parameter (was zero-arg).

Also drops the 'rc' unused-variable warning in v4l2_ioctl_controls
that the iter5 sweep left behind.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/request.h

@@ -80,6 +80,25 @@ struct request_data {
 	 * visible stutter on mpv vaapi --vo=gpu.
 	 */
 	struct cap_pool capture_pool;
+
+	/*
+	 * Per-driver-data cache of the OUTPUT format we've set on the
+	 * V4L2 device (iter5 Track E: was process-global static
+	 * LAST_OUTPUT_WIDTH/HEIGHT, which would race when two libva
+	 * driver_data instances share a process — e.g. Firefox playing
+	 * one tab while Chromium plays another, or two mpv processes
+	 * via the same dlopened backend). Kept per-driver_data because
+	 * the V4L2 device fd is per-driver_data; one fd, one current
+	 * format. Process-global was always wrong, just didn't surface
+	 * until iter5's audit.
+	 *
+	 * See surface.c::CreateSurfaces2 for the consumer pattern (mpv
+	 * probes with small surfaces then re-allocates at real
+	 * resolution; we re-set the OUTPUT format whenever this pair
+	 * changes).
+	 */
+	unsigned int last_output_width;
+	unsigned int last_output_height;
 };
 
 VAStatus VA_DRIVER_INIT_FUNC(VADriverContextP context);